MBIE recently issued guidance for building owners and occupiers on how to assess continued occupancy of a building with a low seismic rating – useful from a number of perspectives…
First, it provides a sound framework for thinking through a ‘stay or go’ decision in the complex world of seismic ratings, earthquake risks and structural weaknesses.
But more importantly, it puts into perspective some of the uncertainty that exists in the world of building science. What do I mean? When we think of engineering, we think equations, formulas and statistics that give certainty to things. We expect defined problems and calculated solutions. We seek exact answers, not generalised statements or opinions. So when it comes to working out how to keep the rate of fatalities from building failures to near zero, we rely on engineers to tell us what design equations will make that happen. Only, that’s just not possible. And the MBIE guidance tells us why.
An indicator of seismic performance
The first clue is that there isn’t a formula, methodology or model that guarantees that no one dies when buildings get hit by seismic forces, including the National Building Standard (NBS) rating. As MBIE points out, the rating was primarily developed to work out whether a building is ‘earthquake prone’ and therefore subject to particular legislative requirements ie legislation that imposes an obligation to strengthen the building above 34% NBS within a certain timeframe. MBIE also reminds us that the rating is not a predictor of building failure in any particular earthquake, nor does a low rating mean a building is imminently dangerous. It usefulness is as ‘an indicator of seismic performance, rather than an exact prediction’.
The second clue is that we need to better understand what building structural elements – should they fail – could lead to fatalities. Because not everything does. The guidance points out that we need to be more concerned about things like columns and beams and hollow core floors than non-structural items like plant and exterior cladding as these tend to carry other (heavy) loads and can do the most damage. If these items are sound, then the risks are reduced.
A testament to building design and construction
MBIE also tells us that many older buildings (some defined as ‘earthquake prone’) survived a 1-in-2000-year earthquake event in Christchurch in 2011. Same with the 2016 Kaikoura earthquake (greater than a 1-in-1000-year event). If the maths applied, then these vulnerable building should have been severely damaged. But many had little or no damage. The actual does not always follow the predicted. This is not to say these building shouldn’t be strengthened (they should as they still pose a heightened risk to life safety in future quakes), but they just may be more structurally sound than our models predict. A testament to building design and construction in NZ.
MBIE also tells us that statistics bear out the fact that loss of life from building damage is extremely low, regardless of building condition. Based on statistical loss of life of one person per 40,000 of population (worst stat), then fewer than two people would be expected to die in the Wellington City CBD during a major earthquake. This low level of risk is not guaranteed of course, but the point is we can expect a lot fewer deaths and injuries than we are likely to predict from buildings themselves.
The guidance from MBIE tells us not to panic if we find out the seismic rating of our building is low (not defined, but more than likely less than 40% NBS). We can probably stay there (assuming no dangerous structural damage already exists), but we need to plan to deal with how to reduce the potential risk now and in the future. Simple works like restraining plant and equipment that can fall from height and cause injury, moving people away from more vulnerable areas, and developing an emergency response plan, are good starts. Ultimately, the building may need to be vacated (or parts of it) to allow seismic upgrade works to be completed. This needs time to in order to plan and carry out. Some upgrades can take one year or more to complete so business decant strategies also need to be worked through.
Outcomes differ
Finally, we know from experience that not all professional advice and engineering reports produce the same outcomes. Engineers are humans who make assumptions in their models based on information they acquire. Differing information leads to differing assumptions which leads to differing results. Second opinions and peer reviews are common in the engineering industry. Often, an assessment is a consensus rather than a defined output. In the same regard, MBIE reminds us we need to see the NBS rating more as a reflection of a performance range than pin-point accuracy. Hence, the NZ Society of Engineering building grading scheme – A-grade being 80% NBS or greater and B-grade being 67% to 80% NBS and C-grade being 34% to 67% NBS. These are broader categories allow the property industry to more simply understand levels of risk without the undue emphasis on a single number.
The Guidance document helps us think about seismic risk more broadly than focusing just on the NBS rating itself. When our building does have a low rating, we need to better understand why and what simple things we can do to further lower risk to occupants before taking the big decision to vacate. And once we do understand the problems with our building, we should realise that they are likely to be stronger and better designed than we give them credit for when life safety is concerned. Fatalities in NZ bear that out. Finally, there is no magic formula for saving lives – risk is a constant that we just have to live with.
